11.2 is the level in the input image that should be mapped to 1.0 in the output image (i.e. the "white point"). If the input image's mean luminance lies noticeably below this 11.2, then the output image will look overall darker.

I'm not familiar with HDR, but AFAIS the white point depends on the current snapshot of the scene. I mean, it would not make sense in general to have a constant white point, because it would result in typical LDR images since you would need to foresee a value for all circumstances. Someone with experience in HDR may confirm or contradict this...

However, the demo scene you are showing us with the screenshots does not look like a typical HDR scene. The brightness of highlights seems relative low, and the fact that 1.0 can be used as white point supports this. In such a scene a constant white point may perhaps be used.

When using the common reinhard tonemapping, you use the scene brightness as variation for the exposure level of the tonemapping operator. You can use something similar for the filimic tonemapping, eg by adding variation to the white scale (or modify other parameters).

To determine the brightness of your scene, render it first (HDR), then sample the image down (eg from 1024x1024=>256=>64=>16=>4=>1) while caluclating the average birghtness/light intensity of the HDR image. Then use the pixel value of the last rendered 1x1 pixel as input (modify by a scalar) for the white space. Utilising the GPU hardware filtering capacities and taking multiple samples per step will result in just a few rendering steps.

Advanced:

Instead of the using only the light intensity of the current scene, take the average of the last X frames.

Unreal Engine has a param for camera component named "Dynamic Range" :

Dynamic Range -> This controls the amount of dynamic range before the tone-mapping curve starts to clip. A setting of 1 would be no dynamic range. Eye adaption happens before the input to Film post so dynamic range is the range after adaption.

So, we end to the answer that it's needed to allow the user to set it.